With the rapid development of electric vehicles worldwide, high energy density, fast charging and high safety have become the core requirements of current power battery technology. Under this demand, the existing technology is gradually relying on large-volume batteries in order to increase the energy density and to shorten the charging time through high charging rates. However, due to the poor thermal conductivity of the battery cell, the internal temperature of a large volume battery cell is usually unevenly distributed under the combined effect of ambient temperature, heat generation of the battery cell and cooling system. The inhomogeneity of battery temperature will lead to differences in the diffusion rate and reaction rate of lithium ions and then form inhomogeneous lithiation in the electrode. At the same time, the non-uniform current distribution generated at higher charging currents during fast charging can further exacerbate the risk of low potential and local near-100% lithiation, resulting in differences in local aging behavior. In this work, a three-dimensional electrochemical thermal model is developed by spatially solving the set of partial differential equations of liquid phase components, potential and temperature in a finite volume method framework. Furthermore, the side reaction models of solid electrolyte film thickening and lithium deposition are combined into the current model to simulate the aging process. The cycling performance and aging mechanism of the cell under different temperature gradients and charge/discharge rates are discussed. The main findings are list in below:
The kinetic parameters and transport parameters of the electrochemical processes inside the cell are affected by temperature, among which the diffusion process of lithium ions inside the electrolyte is more sensitive to temperature distributions. The diffusion and conduction coefficients in the low-temperature region are lower, resulting in difficulty of lithium-ion diffusion in the electrolyte. A higher lithium-ion concentration gradient can be found in the low-temperature region of the negative electrode near the separator.
The inconsistent distribution of impedance caused by the temperature distribution will further cause non-uniform distribution of current density. During the charging process, the current density in the low-temperature region is lower at the beginning and mid-charging stage but increases significantly towards the end of charging. As the charging rate increases, the difference in current density will further increase.
The anode material in the low-temperature region within the simulation results have a higher liquid phase potential than the solid phase potential due to the higher charging current density at the end of charging and the higher liquid phase lithium-ion concentration in the region near the separator, resulting in earlier lithium deposition in this region.
The contributions of this work will help to correct the prediction of Li-ion battery life under the influence of temperature distribution, provide new mechanisms to be considered for the formulation of fast charging strategies, and also provide the optimizations for battery pack design and thermal management schemes.
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